PWM (pulse width modulator) drives are often used to drive electromagnetic actuators or devices with coils. The PWM drive is beneficial because it can efficiently drive heavy inductive loads with little power lost in the PWM drive throughout the entire control range (0 to 100% duty cycle). The coil can act on a mechanical object by means of a magnetic field created by the current in the coil. The magnitude of the magnetic field is directly proportional to the current in the coil so it is important to control or monitor this current. The magnitude of the current can be predicted by dividing the average voltage across the coil by an assumed coil resistance, but because the coil resistance is a strong function of temperature and the temperature can change dramatically as the coil is being driven, this prediction is often insufficient. Many applications use current feedback and closed loop control on this feedback.
When a coil is driven by PWM, the current in the coil is constantly changing. The PWM drive applies a square wave voltage pattern to the coil and hence when the voltage is high, the current is increasing, when the voltage is low, the current is decreasing. The PWM frequency is typically much higher than the response time of the mechanical object being acted on by the coil, so the instantaneous current within the cycle of the PWM is not of value, but the average current is important. Consequently, the feedback variable required is the average current. Very often an Infinite Impulse Response (lag, etc.) filter is used on the feedback signal to yield the average current going to the coil. This inherently adds significant lag to the feedback signal and hence slows the response of the control.
Therefore, there is a need in the art for a method of calculating the average current within the cycle of a PWM that eliminates lag in the feedback signal. By eliminating the lag in the feedback an improved PWM driving circuit would be produced.
Consequently, it is a primary object of the present invention to provide a method of calculating the average current within a PWM cycle using a Finite Impulse Response (FIR) to minimize lag in the feedback signal.
Yet another object of the present invention is to speed up the response of the control of a PWM cycle.
Another object of the present invention is to use a method of calculating an average current within the cycle of a PWM that drives an electrohydraulic valve to improve performance of the valve.
These and other objects, features, or advantages of the present invention will become apparent from the specification and the claims.